As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having a high energy density and a high discharge voltage, on which much research has been carried out and which is now commercialized and widely used.
Based on its external shape, the lithium secondary battery may be generally classified as a cylindrical battery, a prismatic battery, or a pouch-shaped battery. Based on its electrolyte, on the other hand, the lithium secondary battery may be classified as a lithium-ion battery, a lithium-ion polymer battery, or a lithium polymer battery.
As the mobile devices have been miniaturized, the demand of the rectangular battery and the pouch-shaped battery, which have a relatively small thickness, has increased. Especially, much interest is currently generated in the pouch-shaped battery because the shape of the pouch-shaped battery is easily changed, the manufacturing costs of the pouch-shaped battery are low, and the weight of the pouch-shaped battery is small.
Generally, the pouch-shaped secondary battery is manufactured by mounting an electrode assembly, including cathodes, anodes, and separators disposed respectively between the cathodes and the anodes, in a pouch-shaped battery case, constructed in a laminate structure of a resin layer/a metal film layer/a resin layer, and thermally welding a sealed portion, formed along the outer circumference of the battery case, to seal the battery case.
The sealed portion is a region formed by thermally welding the outer circumferential portions of the battery case surrounding the outer surface of the electrode assembly while the outer circumferential portions of the battery case are in contact with each other. It is required for the sealed portion of the battery case to exhibit sealability for preventing external air from being introduced into the battery case during the normal operation of the battery. Also, it is required for the sealing portions of the battery case to release the sealability, when high pressure is generated in the battery due to the abnormal operation of the battery, such as continuous charge and discharge for a long period of time or application of high temperature to the battery, to exhaust gas out of the battery case. Consequently, the sealed portion of the battery case is very important to improve the safety of the battery as well as the efficiency of the battery. However, the sealing function and the gas exhaust function are opposite action mechanisms, and therefore, it is difficult to effectively achieve both the sealing function and the gas exhaust function.
In this connection, there has been restrictively proposed only a structure for releasing the sealed portion when high pressure is generated in a plate-shaped secondary battery; however, there has been not proposed a structure for exhausting pressurized gas out of a plate-shaped secondary battery while maintaining the sealability of the secondary battery.
For example, Japanese Patent Application Publication No. 2005-116235 discloses a technology for forming a specific gas exhaust mechanism in a battery constructed in a structure in which the outer circumferential portions of a laminate film, in which an electrode assembly is mounted, are thermally welded to seal the laminate film. The disclosed technology includes a method of forming partially a non-welded part at the sealed portion, during the thermal welding process, to concentrate the pressure of internal gas to the non-welded part, and a method of deforming the innermost sealing layer of the laminate film such that the innermost sealing layer has an adhesive strength less than that of the remaining region of the laminate film.
However, experiments carried out by the inventors of the present invention revealed that the adhesive force of the sealed portion where the gas exhaust mechanism is formed is small, and therefore, moisture continuously permeates into the battery cell through the sealed portion where the gas exhaust mechanism is formed and an electrolyte continuously leaks from the battery cell through the sealed portion where the gas exhaust mechanism is formed, even during the normal operation of the battery, whereby the safety of the battery is seriously deteriorated. In consideration of a fact that the size of the non-welded part and the size of the adhesive strength must be precisely controlled within a specific range due to the above-mentioned problems, the disclosed technology has a fundamental problem in that it is difficult for the corresponding region to be reliably operated under a high pressure condition requiring the exhaust of gas.
On the other hand, Japanese Patent No. 3,638,765 discloses a battery constructed in a structure in which a plate-shaped electrode assembly is mounted in a sheathing member, and the outer circumferential portions of the sheathing member are attached to each other by thermal welding or an adhesive layer to seal the sheathing member, wherein the sheathing member is sealed while a polymer resin sheet, having a thermal safety higher than that of the sheathing member, is partially inserted between the outer circumferential portions, whereby the sealed portion having the polymer resin sheet exhibits a pressure resistance less than that of the remaining sealed portion. Also, Japanese Patent Application Publication No. 2001-093489 discloses a battery constructed in a structure in which an electrode assembly is mounted in a pouch-shaped battery case made of a laminate sheet, and cathode and anode leads, connected to cathode and anode plates of the electrode assembly, are exposed outward from the sealed portion of a sheathing case, wherein the thermal welding is carried out while a thermal welding resin sheet, having a melting point lower than that of the laminate sheet, is partially inserted in the sealed portion of the sheathing case.
However, the above-mentioned technologies do not secure the sealability and operational reliability of the battery.
On the other hand, technologies for exhausting gas from some devices in one direction are known, although the devices are not plate-shaped secondary batteries. For example, U.S. Pat. No. 5,004,655 discloses a lead battery including a spacer mounted around a battery receiving part, wherein an opening is formed at one side of the spacer, and silicon oil is applied to the opening, thereby preventing external moisture from permeating into the battery while allowing internal gas to be exhausted in one direction. Also, Japanese Patent Application Publication No. 2003-037028 discloses a condenser including a sheathing case constructed in a structure in which a metal film and a plastic film are stacked, wherein a gas exhaust member of an oil sealing structure is mounted at the inside of the sheathing case, and a pin hole for exhausting gas out of the sheathing case is formed at the sheathing case outside the gas exhaust member.
However, the one-way gas exhaust structure of these technologies is applicable to devices that are not seriously affected by moisture introduced into the devices, and therefore, the one-way gas exhaust structure is not structurally applicable to a plate-shaped secondary battery. In conclusion, no attempts have been made to apply the one-way gas exhaust structure to the plate-shaped secondary battery.